The present invention generally relates to the methods and devices for high efficient power conversion by means of an externally heated closed loop regenerative heat engine which utilizes high pressure fluid, preferably carbon dioxide, through at least one scroll expander for the co-generation of shaft power, fluid power or refrigeration.
Currently the state of the art for engines are dominated by internal combustion engines based upon open-loop Otto cycle, Diesel cycle, or Brayton thermodynamic power cycles. Engines based upon these cycles have proven to be sufficiently efficient for many applications and the current state of the art including the benefits and detriments of the various types of engines is discussed in light of the present invention and the objectives of the present invention.
Otto Cycle and Diesel Cycle engines are used primarily for application in automobile, airplanes and other low cost applications (lawn mowers, pumps, etc. . . . ). These types of engines (two and four stroke engines) are efficient, lightweight, and fairly inexpensive to manufacture. Generally, in the last eighty years there has been much more focus in improving the designs and efficiencies for these types of engines by the various industries needing a cheap efficient means for converting power. There are significant limitations associated with using these types of internal combustion engines, including: a efficiencies of approximately 20% to 30%; a limited type of fuel associated with each type of engine; serious vibration and noise associated with cams, camshafts and piston rods; power density limitations; significant green house and carbon fuel emission associated with internal combustion engines; and limitations associated with operations of an internal combustion engine at limited air density environments (Brayton cycle turbines have this limitation as well).
In an internal combustion engine, the working fluid is primarily air. Heat through combustion is created by injecting and burning fuel with the working fluid at the proper location and at the proper time in the engine's cycle. This enables the working fluid to be expanded, which in part, produces work. While these engines are well understood and well developed, it is also known that these engines produce much less power than their theoretical limits due to the limitations association with the friction, heat loss and the timing associated with the combustion of the fuel and air mixture within the cylinder of an engine block.
These limitations can also limit the ability to control the quality of combustion and the range of air to fuel mixtures that can be ignited. The level of power available from these types of engines (Otto and Diesel cycle engines) is proportional to the mass flow of air passing through the engine itself. It is well known that these engines decrease delivered power as the atmospheric density of air decreases with altitude and the air temperature increases because the net mass flow of air available to the engine decreases.
In many applications, engines must operate in an environment with reduced atmospheric density. There is a decrease of power availability due to atmospheric density that is noted by the prior art and addressed in U.S. Pat. No. 7,284,363 which discloses a means for power generation for airborne vehicles operating at an altitude of 50,000 feet (above sea level). The various descriptions, in the '363, for closed loop engines converting power are limited to generic claims utilizing a Brayton or Rankine cycle type engine. The description of the working fluid expansion in the closed loop is done by means of a turbine, in both the Brayton and Rankine cycle mode of operation.
The present invention provides an engine that is capable of operating in a variety of thermodynamic cycle modes, including; Rankine, Brayton, or a supercritical cycle similar to U.S. Pat. No. 3,237,403 discussed below. The mode of operation is dependent on several factors, including: the environment in which the engine is operating; the type of working fluid used in the engine; the type of heat source utilized; and manipulation of the working fluid's temperature and pressure. The present invention takes advantage of the close loop system and is not limited to applications less than 50,000 feet. Additionally, the present invention provides a means for accomplishing power conversion in a small lightweight package with a high power density that can be utilized in numerous environments.
It is one objective of the present invention to be able to operate as a closed loop Rankine thermodynamic cycle for maximum efficiency and, depending on the conditions in which the engine is operating and the type of heat source used by the engine, be able to operate in other thermodynamic modes, like a Brayton cycle. Heat supplied to the working fluid is provided external to the engine and is transferred to the working fluid through means of a heat exchanger, such as an evaporator or boiler, thereby eliminating inefficiencies associated with integrating heat addition within the engine.
Generally the heat source will be something that is created through combustion of fossil fuels. External combustion or heat addition is an excellent means for increasing the efficiency of power generation without compromising the method of the heat addition. External combustion also greatly reduces the amount and type of green house gasses and pollution emitted by the engine. One objective of the present invention is to provide an engine that is able to use a variety of fuels or heat sources.
There are several externally heated engines in the prior art that are based upon the Stirling, and Ericsson thermodynamic power cycles. These examples in the prior art follow both open-loop and closed-loop thermodynamic cycle. There are also a number of mechanical and fluidic embodiments of these cycles in the prior art. From a theoretical stand point both the Stirling and Ericsson cycles potentially achieve efficiency near the absolute limit, defined by the efficiency of a Carnot cycle; however, in actual practice these cycles require isothermal compression and expansion of the working fluid. The physical means for achieving an isothermal process in compression and expansion is bulky, involves friction losses, and is limited by the power rate that can be achieved with heat exchangers. This has proven to make these types of heat engines heavy for the power they produce and do not achieve their desired theoretical efficiency.
For example, U.S. Pat. No. 7,124,585 discloses a scroll type expander having an integrated heating surface for the exchange of thermal energy to work output as a means for power conversion in this Stirling cycle type engine. This particular invention, besides having the limitation described above, has limitations associated with capturing or exchanging thermal energy integrated with an engine bloc of the system. In creating this type of engine, which has high theoretical efficiency, there is in reality several impracticalities for producing a small, lightweight, high power engine as described in the present invention, such as size and power output limitations.
In the present invention, the heat source is provided independent of the engine block or work output means and therefore provides more flexibility for the design and power output of the engine. Additionally by divorcing the heat generation from the engine block or power producing portion of the engine, heat sources that use carbon fuel consumption can be greatly enhanced with respect to the efficiency associated with complete combustion, heat transfer of combusted fuels, increase in the type of fuel consumed and a cleaner more easily managed fuel exhaust. It is one of the objectives of the present invention to provide an engine that is flexible with respect to the types of fuels that will be used for combustion or the type of heat source used on a working fluid in the closed loop. Currently, governmental and societal demands have been trending toward a need for an engine that is flexible with respect to the type of fuel consumed or used by engines.
In one embodiment of the present invention, the engine operates using the well proven and understood Rankine cycle. By taking advantage of the phase change in the closed loop, its efficiency is comparable to that achievable by Stirling and Ericsson cycle engines, but its power capability is far higher because it is not limited by isothermal compression and expansion. A good description of the power efficiency associated with the present invention is found in U.S. Pat. No. 3,237,403 issued to Feher in 1966 which discloses a device and method for using a supercritical fluid in a heat engine. The patent discloses the benefits associated with an external engine operating in a Rankine cycle. The description of the closed loop system anticipates a turbine or possibly a piston engine for expanding the high temperature high pressure working fluid (Col. 2, line 2-5). The patent, while describing the benefits of using a supercritical working fluid at a low cycle pressure substantially above critical pressure and a temperature below critical temperature, still lacks detail on how to effectively accomplish this process for a high pressure high temperature working fluid in a relatively small, lightweight package. The means for expanding or the method for expanding the working fluid in the claims are not disclosed in any detail, other than anticipation or use of a turbine.
In addition to a generic description of the process and the benefits associated with operating an engine at prescribed temperatures and pressures, the '403 concedes the “various mechanical components of the system are quite conventional in type but the components must be specially designed and built to operate properly under special conditions such as pressure, pressure ratio, high density of fluid passing through the turbine, and temperature and pressure limits in the regenerator, evaporator, condenser, etc.” The present invention addresses these limitations and actually describes in detail an engine that can operate in the mode described by the '403 as well as parameters beyond the scope of the '403.
In the detailed descriptions and claims of the '403, there was very little detail provided for the type of engine that was to be used in the application of the '403 patent. With the exception of calling for a “turbine”, the prior art relating to this type of engine concept do not address the means for power conversion addressed by the present invention.
In a Rankine cycle, as in one embodiment of the present invention, the engine's working fluid changes phase from liquid phase to gaseous phase after heating of the working fluid and from a gaseous phase to a liquid phase with the removal of heat. In a Brayton cycle the working fluid does not change phase. Its working fluid remains a gas or super-critical fluid throughout the cycle. For working fluids like air, Helium, or Nitrogen this lack of phase change is appropriate since the pressures and temperatures required to enable a phase change are impractical.
The present invention is able to take advantage of a working fluid that undergoes a phase change in a closed loop portion of the engine. In a Rankine cycle the working fluid is cooled to a liquid phase before a pump or means of pressurizing is used to increase its pressure prior to heating of the working fluid. The expansion of the working fluid in this type of system provides for a much more efficient thermodynamic cycle than Otto or Diesel cycle engines and most Brayton thermodynamic power cycles.
As noted in the prior art, the work to compress a liquid is far less than the work required to compress a gas or super-fluid. The gains associated with less work input to compress the fluid will result in more net power; therefore, reducing the work required to pump the working fluid to the cycle's high pressure increases the net power produced by the engine. For this reason, Rankine cycle engines tend to be more efficient than Brayton cycle engines.
One objective of the present invention is to provide a closed loop operating system in which the working fluid in a low temperature and low pressure portion of the loop can either be a liquid (phase change—Rankine cycle), vapor (no phase change—Brayton cycle) or a supersaturated high density fluid. The ability to operate an engine in various thermodynamic cycles is a tremendous advantage in applications for which the engine can operate. Advantages for operating in different thermodynamic modes include; various working environment in which the engine can operate, various working fluids can be utilized with little or no alterations of the basic design, and various heating sources can be utilized to heat the working fluid. These advantages in the present invention are not found in the prior art and provide for a flexible operating engine for numerous applications.
The selection of working fluids has some but very little impact on the theoretical potential of efficiency for the various thermodynamic cycles in which the engine operates, and primarily the operating temperatures and pressures of the cycle control this feature. Many types of working fluids have been used in Rankine cycle type engines in the past, including; water, nitrogen, carbon dioxide (CO2), propane, and various other organics. The working fluid to be used in a closed loop thermodynamic engine with an external heat source will depend on the range in which the heat source is able to produce heat and a heat sink source of a condenser in the closed loop. In the present invention, the engine is able to operate using various types of working fluids and the choice of the fluid would be dictated by the working environment in which the engine operates or the type of heat source to be used.
The selection of a working fluid is used to address the practical needs to transfer heat into the engine and to handle the working fluid as it changes phase. The present invention engine in one embodiment uses carbon dioxide (CO2) as its working fluid due to its stable and non-reactive characteristic to very high temperature and remains a liquid to a very low temperature. This feature of CO2 provides the potential for very high thermodynamic efficiency. There are practical challenges to using CO2 as a working fluid because of its high critical pressure, yet relatively low critical temperature. Many of the features of the present invention address this particular technical challenge. The present invention also takes advantage of CO2 thermodynamic properties, independent of its function as a working fluid for the Rankine cycle, for co-generation of refrigerant power and as a hydraulic power media for transferring mechanical power to various applications.
By using an external combustion process the in-efficiencies from integrating heat addition within the engine are eliminated. It allows the heat to be added to the cycle in a manner that does not compromise either the function of the engine or the efficiency and quality of the heat being provided. If the source of heat to the engine comes from the combustion of fuel and air, the control of the combustion can be optimized to maximize the heat provided and does not have to be constrained to the needs of the engine or its thermodynamic system. For example, to extract power from the engine, the pressure of the working fluid usually has to be maximized. For extracting heat from combustion, the pressure of the fuel and air mixture is not as critical and often not desired to be too high.
The external heat addition of the invention allows the needs of the power cycle to be addressed in design, and remain independent for the needs of heat addition. The mass flow of working fluid in the engine of this invention is also independent of the external environment and independent of the external heat addition. This means that power density of the engine can be increased by increasing the mass flow of working fluid through the engine. The fact that the working fluid of the thermodynamic cycle of the invention engine follows a closed-loop allows a separation of the power means of the engine from the heat addition means for the engine. It also allows the tailoring of the engine's working fluid to maximize power density and other important design considerations not possible if the working fluid is restricted to air in the engine's environment. One simple benefit of this arrangement is that the available power from the engine is not strongly dependent upon the density of the air of its environment. The power available from the invention engine is only dependent upon air density to the extent the external heating is dependent upon air density.
Most if not all of the prior art that takes advantage of an external heat source applied to a closed loop system describes expansion of the high pressure working fluid through a turbine type device. Turbines are an excellent means for converting thermal energy into mechanical energy with only a couple limitations. Turbines condition the flow of the working fluid by converting pressure into flow velocity to convert momentum into useful work. This requires the turbine to operate at high rotational velocity to achieve desired efficiencies of energy conversion. This results in the drive shaft, connected to the turbine, to also have a high rotational speed. A transmission device is required to make the shaft speed of the turbine useful for various applications. The present invention is a positive displacement device and converts pressure into work by direct expansion of pockets or discrete volumes of working fluid. The expansion of discrete volumes of working fluid within one or more scroll expanders enables a shaft output to operate more efficiently over a wide range of rotational velocities. By providing a means for obtaining a range of rotational speeds without losing efficiency provides a user with a wide variety of outputs or speed conditioning for useful applications. For example, rotational speeds needed for a generator, hydraulic pump or motor can be easily produced from the same scroll expander with little or no modification to the closed loop system.
Another objective of the present invention is to provide a means for converting high pressure working fluid into useful work in a small, lightweight package. A turbine is designed to concentrate the high pressure working fluid near or at the external lines of the turbine casing. The center portion of the turbine is occupied with the rotational element of the turbine itself, including a shaft, bearings, and seals. With high pressure fluid flow at the outer portions of the turbine casing, additional weight is necessary for maintaining the turbines integrity. The present invention is able to minimize the effects of high pressure working fluids loading a casing or engine block in which the scroll expander is placed. The scroll expander receives the high pressure working fluid at the center of the scroll expander with expansion of the working fluid decreasing as the working fluid travels through the scroll expander. The periphery of the casing or engine block is presented with a relatively lower pressure working fluid and therefore less weight is needed to maintain the integrity of the closed loop. This center out pressure reduction in a scroll expander of the working fluid results in a lighter and more compact thermal expansion device or engine block.